There is an increasing interest in electric light bulbs which do not make use of incandescent filaments, since filament-based light bulbs are considered to be inefficient and energy hungry. Indeed, recent legislative changes mean that traditional incandescent light bulbs are being phased out in many parts of the world. One existing replacement for the incandescent light bulb is the compact fluorescent light bulb.
Solid state lighting, for example light emitting diode (LED) or organic light emitting diode (OLED) based retrofit lamps, offer superior performance over compact fluorescent lamp (CFL) based retrofit lamps in terms of efficiency, instant light output, light quality, and lifetime. The main barrier to penetrate the market is product cost, since the shop price of today's LED-based lamps can be up to 10 times that of CFL lamps.
A key element of an LED lamp assembly is the LED light source. The luminous efficiency, measured in lumen per watt, has been improved significantly over the last 10 years, and continues to increase further to levels of 250 lm/W for white light LEDs, with potential for further improvement.
Another strong advantage of using LED light sources is that they offer superior lifetime since the only failure mode is a slow depreciation of the outcoupling optics of the light source.
In order to exploit the enormous advantages of LED in terms of its luminous efficiency and its potentially superior lifetime, it is necessary to maintain the LED device strictly below its specified maximum temperature.
Although the LED offers excellent luminous efficiency over alternative light source technologies one strong disadvantage of any solid state light source is that there is no radiation of energy in the form of infrared radiation so that any power losses inside the light source component has to be propagated to the environment purely by heat conduction.
Another aspect of maintaining the LED at or below a desired temperature is that the efficiency of the device degrades with increasing temperature, thereby increasing further the device temperature, assuming the power provided by the power supply is kept at a constant level.
The fast and dynamic improvement of LED efficiency can be utilized only if the manufacturer of solid state light source (SSL) based lamp assemblies can change their product to accommodate new generations of SSL components. This requires normally a redesign of components within the power supply or driver unit inside the SSL lamp assembly due to changing requirements of the SSL. Such redesign tasks are resource and time intensive and increase heavily the total cost for the manufacturer of SSL lamp assemblies.
In order to maintain the light source device at and below a strict temperature limit requires the SSL lamp assembly to be carefully optimized with respect to its thermal performance and characteristics. The system should be designed that under all conditions, including worst case conditions, the SSL chip is maintained below its maximum specified temperature. This may require bulky cooling parts in order to ensure a low thermal resistance between the SSL chip and its surroundings. The cooling of the SSL chip is a major contribution to the product cost and can be a factor in the reluctance to accept SSL lamp assemblies by the end user.
One technique used in previously-considered power supply circuits for SSL lamp assemblies is to use control integrated circuits (ICs) which operate a specific power electronics architecture at an operating point which is set by a combination of references inside the control IC, the values of external components, the actual condition of the input power as well as the electrical characteristics of the SSL device. All these factors produce a statistical variance across of power delivered to the SSL device over a range of manufactured SSL lamp assemblies. The outer limits of the distribution of power dissipated within the SSL device as well as the variance of the ambient temperature in which the lamp assembly is to be operated defines the requirements for the worst case thermal conditions for which the system has to be operated such that all components are within the safe operating area that guarantees the rated system lifetime.
Lamp assemblies have to comply with safety requirements which require the system to stay within temperature limits. In order to guarantee safety limitations as well as to guarantee lifetime specifications state of the art control ICs for SSL driver units offer a temperature shutdown function which turns the system off whenever a specific threshold is reached at a temperature sensor. The shutdown threshold is at a fixed temperature range between maximum and minimum values which result out of manufacturing tolerances.
In previously-considered solutions, the system remains in shutdown mode once the over temperature condition was met at one time.
In order to avoid any irregular shutdown, or, in case there is no thermal shutdown, in order to maintain a certain temperature maximum at any time in any rated operating condition, the thermal design of a SSL lamp assembly must foresee a temperature headroom. This causes additional substantial material cost which is not effective in most cases because the statistical limits are not met in most cases.
In previously-considered control ICs for SSL lamp assembly power supply circuits the optimization of the thermal subsystem is strongly limited by the large variance in electrical thermal and optical parameters and requires the implementation of redundancy in the cooling system.
It is desirable to provide a controller and driver circuit that is able to adapt the power delivered to an SSL component in order to maintain the maximum temperature of the SSL component at or below a specified level. Such a design can serve to reduce the reserve which is needed in the thermal design for worst case conditions, thereby reducing the cost of the cooling system of the SSL lamp assembly.
It is also desirable to provide a controller, driver circuit and SSL lamp assembly that can serve to reduce the cost of adapting lamp assemblies to feature new SSL components.